It is generally known that antenna performance is dependent on the antenna size, shape and the material composition of certain antenna elements, as well as the relationship between the wavelength of the received/transmitted signal and antenna physical parameters (that is, length for a linear antenna and diameter for a loop antenna). These relationships and physical parameters determine several antenna performance characteristics, including: input impedance, gain, directivity, polarization and radiation pattern. Generally, for an operable antenna, the minimum effective electrical length (which for certain antenna structures, for example antennas incorporating slow wave elements, may not be equivalent to the antenna physical length) must be on the order of a quarter wavelength (or a multiple thereof) of the operating frequency. A quarter-wavelength antenna limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter and half wavelength antennas are the most commonly used.
The radiation pattern of the half-wavelength dipole antenna is the familiar omnidirectional donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical antenna gain is about 2.15 dBi.
The quarter-wavelength monopole antenna placed above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but when disposed above a ground plane the antenna performance resembles that of a half-wavelength dipole. Thus, the radiation pattern for a quarter wavelength monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
Printed or microstrip antennas are constructed using the principles of printed circuit board processing, where conductive layers on one or more dielectric substrates are patterned, masked and etched to form the antenna elements. The conductive layers or interconnecting vias serve as the radiating element(s). These antennas are popular because of their low profile, ease of manufacture and low fabrication cost.
One such antenna is the patch antenna, comprising in stacked relationship, a ground plane, a dielectric substrate, and a radiating element overlying the substrate top surface. The patch antenna provides directional hemispherical coverage with a gain of approximately 3 dBi. The patch antenna exhibits a relatively bandwidth and low radiation efficiency, i.e., the antenna exhibits relatively high losses within its radiation bandwidth. Patch antennas can be stacked or disposed in a single plane with a predetermined spacing therebetween to synthesize the desired radiation pattern that may not be achievable with a single patch antenna.
The common free space (i.e., not above a ground plane) conventional loop antenna, with a diameter of approximately one-third the operative wavelength, also displays the familiar omnidirectional donut radiation pattern along the radial axis, and exhibits a gain of about 3.1 dBi. At 1900 MHz the loop antenna has a diameter of about two inches. The typical loop antenna impedance is about 50 ohms, providing good matching characteristics to the feed transmission line.
The burgeoning growth of wireless communications devices and systems has created a need for physically smaller, less obtrusive and more efficient antennas that are capable of wide bandwidth and/or multiple resonant frequency operation. As the physical enclosures for pagers, cellular telephones and wireless Internet access devices shrink, manufacturers continue to demand improved performance, multiple operational modes and smaller sizes for today's antennas.
Smaller packaging envelopes may not provide sufficient space for the conventional quarter and half-wavelength antenna elements. Also, as is known to those skilled in the art, there is a direct relationship between antenna gain and antenna physical size. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennas.
Given the advantages and efficiencies of a quarter wavelength antenna, prior art antennas have typically been constructed with elemental lengths on the order of a quarter wavelength of the radiating frequency. These dimensions allow the antenna to be easily excited and to be operated at or near a resonant frequency, thereby limiting the energy dissipated in resistive losses and maximizing the transmitted energy. But, as the resonant frequency decreases, the resonant wavelength increases and the antenna dimensions also increase.
As a result, some antenna designers have turned to the use of so-called slow wave structures where the physical antenna dimensions do not directly represent the effective electrical length of the antenna element. As discussed above, but for the use of such slow wave structures, the antenna length must be on the order of a half wavelength to achieve the beneficial radiating properties. The use of a slow wave structure as an antenna element de-couples the conventional relationship between physical length and resonant frequency. The effective electrical length of the slow wave structure is greater than it's actual physical length, as shown in the equation below.le=(εeff1/2)×lpwhere le is the effective electrical length, lp is the actual physical length, and εeff is the dielectric constant (εr) of the dielectric material on which the slow wave structure is disposed. Generally, a slow wave structure is defined as one in which the phase velocity of the traveling wave is less than the free space velocity of light. Slow wave structures can be used as antenna radiating and non-radiating elements.
A meanderline transmission line is one example of a slow wave structure, comprising a conductive pattern (i.e., a traveling wave structure) over a dielectric substrate, which in turn overlies a conductive ground plane. An antenna employing a meanderline structure, referred to as a meanderline-loaded antenna or a variable impedance transmission line (VITL) antenna, is disclosed in U.S. Pat. No. 5,790,080. The antenna consists of two vertical spaced-apart conductors and a horizontal conductor disposed therebetween, with a gap separating each vertical conductor from the horizontal conductor. The antenna further comprises a meanderline variable impedance transmission lines bridging the gap between the vertical conductor and each horizontal conductor. Generally, a meanderline structure is one comprising a non-linear or winding conductive element disposed over a dielectric substrate.
Using these meanderline structures, physically smaller antenna elements can be employed to form an antenna having, for example, quarter-wavelength characteristics, although the antenna physical dimensions are less than a quarter-wavelength. Although the meanderline-loaded antenna offers desirable attributes within a smaller physical volume, as hand-held wireless communications devices continue to shrink, manufacturers continue to demand even smaller antennas, especially those that are easily conformable into the available volume. Meanderline-loaded antennas, such as those set forth in the above referenced patent, are typically not easily conformable. Also, the antenna should desirably exhibit wide-bandwidth performance or have one or more resonant frequencies (thus having the effect of wide bandwidth performance). Further, the antenna must exhibit the radiation pattern required by the intended application. The prior art meanderline antennas may not generally exhibit these characteristics.